1. Field of the Invention
This invention relates to a circuit-constituting unit forming a distribution circuit or the like, and to a method producing the same.
2. Background Art
As conventional means for distributing electric power to electrical loads from a common power source, there is known an electric connection box of the type in which a plurality of bus bar boards are stacked together to form a distribution circuit, and fuses and relay switches are incorporated in this circuit.
In recent years, in order to achieve a compact design and high-speed switching control of such an electric connection box, there has been developed the type of circuit-constituting unit in which instead of using the relays, circuit elements, such as FETs, are provided.
For example, JP-A-10-35375 discloses an electric connection box which comprises a bus bar board, forming a current circuit, FETs (semiconductor switching devices), incorporated in the current circuit, and a control circuit board for controlling the operation of the FETs, and the bus bar board and the control circuit board are arranged at two (upper and lower) stages in spaced relation to each other, and the FETs are disposed between the two boards, and drain terminals and source terminals of the FETs are connected to the bus bar board while gate terminals of the FETs are connected to the control circuit board.
JP-A-2001-268785 discloses a power distributor in which an insulating casing, made of a synthetic resin or the like, is molded around a plurality of bus bars, arranged generally in a common plane, to provide an integral construction, and semiconductor switching devices, such as FETs, are mounted on suitable bus bars to form a power circuit, and a control circuit board is provided in generally parallel, spaced relation to the insulating casing, and this control circuit board is connected to the semiconductor switching devices, and the driving of the semiconductor switching devices is controlled by a control circuit incorporated in the control circuit board.
In the electric connection box, disclosed in JP-A-10-35375, there are needed at least two boards, and besides there must be secured a space for allowing these boards to be arranged in a three-dimensional manner in spaced relation to each so that the FETs can be provided between the two boards. Therefore, as a result of the introduction of the FETs, this electric connection box can be made more compact than the conventional electric connection box of the relay type, but the overall construction thereof is complicated, and can not be made sufficiently compact in size, and particularly there has been encountered a problem that the height thereof must be reduced.
And besides, in the above electric connection box, the FETs are provided between the bus bar board and the control circuit board, and therefore heat, generated from the FETs, is liable to remain in the space between the two boards, and therefore it is necessary to adopt a complicated structure for radiating this heat.
Furthermore, in the above electric connection box, the drain terminal and source terminal of each FET are connected to the lower bus bar board while the gate terminal thereof is connected to the upper control circuit board, and therefore the operation for assembling the electric connection box is complicated, and it is difficult to effect this assembling operation in an automated manner, and it has been desired to improve these situations.
In the power distributor described in JP-A-2001-268785, the bus bars, arranged generally in the common plane, are united by the insulating casing, and therefore it is simpler in construction than the conventional electric connection box, but a thin design thereof is limited. Therefore, the Applicant of the present application has invented a technique of producing a circuit-constituting unit by bonding the bus bars to a surface of a sheet-like insulative base member.
One example thereof is shown in FIG. 20. In this Figure, a number of bus bars, including input terminal bus bars 111 and output terminal bus bars 112, are bonded to one side (lower surface in the illustrated example) of the thin sheet-like insulative base member 120. FET-mounting windows 122 are formed through the insulative base member 120, and each FET 30 is mounted on the corresponding input terminal bus bar 111 and output terminal bus bar 112 through the window 122 from the upper side (in this Figure) in such a manner that the FET straddles a gap between the two bus bars.
The FET 30 includes a body 32 of a generally rectangular parallelepiped shape, and terminals (only a source terminal 34 is shown here), and a drain is exposed at a lower surface of the body 32. The FET body 32 is mounted on the input terminal bus bar 111 in such a manner that the drain is held in contact with the upper surface of this bus bar 111, and the source terminal 34 is mounted on the output terminal bus bar 112.
Such a circuit-constituting unit can form a distribution circuit, including the FETs 30, with a thin structure. More specifically, when the FET 30 is turned on, electric power, inputted to the input terminal bus bar 111, is fed to the output terminal bus bar 112 through the FET 30, and when the FET 30 is turned off, the electrical connection between the bus bars 111 and 112 is interrupted.
In such a circuit-constituting unit of the thin structure, however, the insulative sheet 120 is liable to be flexed at those regions (for example, that portion between the input terminal bus bar 111 and the output terminal bus bar 112 in FIG. 20) where the bus bars are not bonded to this insulative sheet 120 (see dots-and-dash lines in this Figure), and therefore there is a fear that those portions of the FET 30, soldered to the bus bars 111 and 112, are separated therefrom.